Answer:
The Zimmerman Telegram, that the German Empire had sent to Mexico to invade the U.S. They had also been sinking U.S. civilian ships, which they claimed had weapons on board, which they had. So their claim was right and continued. Great Britain caught the Zimmerman Telegram by it having to go through London for it to them reach Mexico. So when Great Britain showed it to the U.S. they weren't very happy and joined the war.
Step-by-step explanation: